Acidification and recovery of aquatic ecosystems in south central Ontario, Canada: regional application of the MAGIC model

Abstract. The dynamic model MAGIC was applied to 25 lakes in south-central Ontario, Canada using a regional modelling methodology. Soil and lake water chemistry for each lake catchment was simulated for the period 1850–2050. Sulphate (SO₄^2–) deposition forecasts were derived from recently proposed emission reductions, which correspond roughly to a 50% reduction in SO₄^2– deposition by 2010 from the 2000 baseline. Changes in SO₄^2– deposition had a significant impact on lake chemistry. Simulated lake water chemistry showed a recovery potential under the current deposition scenario; by 2050 concentration levels recovered to values predicted for the early 1900s. Moreover, simulated future lake water chemistry showed significant recovery compared to 1975 levels. However, although regional simulations predict that base cation losses have decreased in recent years, soils in the region will continue to acidify with Ca²⁺ losses dominating depletion of the exchangeable pool. Base cation losses from the exchangeable pool are currently buffering lakes against the impacts of acid deposition; ultimately base cation inputs into the lakes will decrease as exchangeable base cation pools become depleted. Further emission reductions are necessary to ensure continued recovery from acidification.

Keywords: regional dynamic model, sulphate, acid neutralising capacity, critical loads, lake chemistry, soil base saturation